The UN said up to 50 people were killed in air strikes on medical facilities and schools in rebel-held Syrian towns on Monday, including a hospital supported by medical charity Doctors Without Borders.

The United Nations said air strikes targeted at least five medical facilities and two schools in northern Syria's Aleppo and Idlib provinces, killing nearly 50 civilians including children.

The air raids prompted outrage among Western powers, with US officials laying the blame squarely on the regime of Syrian President Bashar al-Assad and its allies in Moscow.

"That the Assad regime and its supporters would continue these attacks, without cause and without sufficient regard for international obligations to safeguard innocent lives, flies in the face of the unanimous calls by the ISSG [International Syria Support Group]... to avoid attacks on civilians," the State Department said.

It said such action "casts doubt on Russia's willingness and/or ability to help bring to a stop the continued brutality of the Assad regime against its own people".

Doctors Without Borders (also known as Médecins sans frontières, or MSF) said the hospital it supported in Murat al-Numan in the northern province of Idlib was “deliberately” struck by four missiles.

"There were at least seven deaths among the personnel and the patients, and at least eight MSF personnel have disappeared, and we don't know if they are alive," said Mego Terzian, head of the charity’s French branch.

MSF said the hospital near Murat al-Numan in the northern Syrian province of Idlib was struck by four missiles during two air raids.

"The author of the strike is clearly […] either the government or Russia," Terzian said, adding that it was not the first time MSF facilities had been targeted in the country.

The hospital, which has 54 staff and holds 30 beds, is financed by the medical charity. MSF also supplies medicines and equipment to the facility.

"This is a deliberate attack against a health establishment," said Massimiliano Rebaudengo, MSF head of mission. "The destruction of this hospital deprives about 40,000 people of healthcare in this conflict zone."

The Syrian Observatory for Human Rights, which tracks violence across the country, said another strike hit the National Hospital on the town’s northern edge, killing two nurses.

Residents blamed Russian strikes, saying the planes deployed were more numerous and the munitions more powerful than the Syrian military typically used.

Children’s hospital hit near Turkish border

In a separate attack, at least 14 civilians were killed when missiles hit a children's hospital, a school and other locations in the rebel-held Syrian town of Azaz near the Turkish border on Monday.

According to a medic and two residents, at least five missiles hit the hospital in the town centre and a nearby school, where refugees fleeing a major Syrian army offensive were sheltering.

"We have been moving scores of screaming children from the hospital," said medic Juma Rahal. At least two children were killed and ambulances ferried scores of injured people to Turkey for treatment, he said.

Turkey slams 'barbaric' attacks against civilians

A resident said another refugee shelter south of the town was also hit by bombs dropped by jets believed to be Russian.

Russian warplanes have conducted air strikes in Syria since September 30 in support of President Bashar al-Assad's forces.

Kurdish advance triggers Turkish shelling

The town of Azaz lies on a corridor stretching from the Turkish border to the contested city of Aleppo. The corridor is the last supply line for Syrian rebels stranded in Aleppo.

Though it is currently held by Syrian rebel forces, Azaz faces multiple threats, including from Assad loyalists in the south and Syrian Kurds in the west.

A recent advance by Kurdish YPG forces prompted cross-border shelling from Turkey, which continued for a third day on Monday.

The YPG’s advances have raised deep concern in Ankara, which accuses the Syrian Kurdish group of ties to the outlawed Kurdistan Workers Party (PKK), based in Turkey.

Turkish officials, who support the rebels controlling Azaz, have said they will not allow Kurdish forces to seize key areas along the border.

The move drew fierce criticism from Russia on Monday, which said Turkey’s “provocative” shelling was “creating a threat to peace and security in the Middle East and beyond”.